1996A&A...312..525P

A method to compute a NLTE model of the atmosphere of a type Ia supernova (SN Ia) near maximum light is presented. The determination of the level populations is carried out using detailed atomic models, and including all important contributions to the rate equations: Thomson scattering, bound-free (from ground and excited levels) and free-free opacities, line absorption and emission processes. Dielectronic recombination is included. The spherical radiation transfer is solved at up to 400 frequency points and 41 depth points. Finally, a synthetic spectrum is computed using a formal integral solution of the transfer equation based on a spatial microgrid. It is found that the SN atmosphere is electron scattering-dominated, and that the high velocity of the apparent photosphere (∼8000km/s) is due to the pseudo-continuum opacity created by the thick line forest which blocks the flux in the UV and optical part of the spectrum. Increasingly more sophisticated treatments of the process of flux blocking in the UV (line blocking) are discussed. The necessity of treating the far-UV flux correctly is demonstrated. Line blocking in the region 800-1300A reduces the photoionization from the excited levels of several important ions (e.g. Fe II, Co II, Si II, Ca II), thus decreasing the overall degree of ionization. This effect is clearly seen in the synthetic emergent spectra. Synthetic spectra obtained with the various methods adopted for line blocking are shown, and compared to one another. When line blocking is properly treated, the synthetic spectrum reproduces well the spectrum of the `normal' SN Ia 1992A from the UV to the near-IR.